DML and Constraints on Views

Performing DML Operations on a view:
DML operations can be performed upon a table through view.
Rules to Follow:
Ø  A Row can be removed from a view unless it contains:
    • Group Function
    • A Group by Clause
    • The Rownum Pesudo column
    • The Distinct key word
    • The columns defined by expressions
Ø  Data can be added through a view, unless it contains any of the above rules and there does not exist not null columns, and without default value.
Ex:
  • Create view InsertDept(DeptId, DeptName, Place)
            As Select Deptno, Dname, Loc From Dept;
Inserting Data Into Dept Table Using The InsertDept View:
Ex:
  • Insert into InsertDept(DeptID, DeptName, Place) Values(50, ’ADMINISTRATION’, ’DELHI’); 
Updating Data in Dept Table The InsertDept View:
Ex:
  • Update InsertDept Set place = ’MUMBAI’ where DeptID = 50;
Deleting Data From Dept Table Using the InsertDept View:
Ex:
  • Delete From InsertDept where DeptID = 50;
Ø  Many times these transactions look consistent but it is not true, hence care should be taken when implementing them.
Using with Check Option Clause:
Ø  To ensure that DML on the view stays within the domain of the view we use the with check option clause.
Ø  Views make it possible to perform referential integrity checks.
Ø  Using views we can enforce constraints at database level.
Ø  Using Views we can protect the data integrity, but the use is very limited.
Ø  With Check option clause specifies that inserts, & updates performed through the view are not allowed to create rows that the view cannot select.
Ø  Views allow integrity constraints and data validation checks to be enforced on data being inserted or updated.
Ex:
  • Create or Replace view EDept30
            As Select * From Emp where Deptno = 30
            WITH Check option constraint EDept30ChkView;
  • Create or Replace view Emanage
            As Select * From Emp where Job = ‘MANAGER’
            WITH Check option constraint EmangerView; 

Applying With Read Only Option:
Ø  By adding with read only we can ensure that no DML operations are executed through view.
Ø  An Attempt to perform a DML operations result in oracle server error.
Ex:
  • Create or Replace view EDeptRead ( EmpID, Name, Designation )
            As Select Empno, Ename, Job From Emp where Deptno = 20
            WITH Read Only;
View Constraint:
Ø  In practically oracle does not enforce view constraints, but operations on views are subject to the integrity constraints defined on the underlying base tables.
Ø  Constraints on views can be enforced through constraints on base tables.
Restrictions on View constraints:
Ø  The view constraints are subset of table constraints.
Ø  Only UNIQUE, Primary key and FOREIGN key constraints can be specified on views.
Ø  The check constraint is imposed using WITH Check Option.
Ø  As view is a logical table the constraints on view are not highly considered in practice.

Ø  The view constraints are adopted for maintenance easiness.